Fix deleted messages remaining visible in chat

[TiaraBasori]

Summary

Invalidate the transcript memoization when messages are removed so deleted messages disappear immediately instead of remaining visible until the chat is reopened.

Root cause

The message list is mutated in place during deletion. Transcript caching only checked list identity, so it reused stale entries after the view model notified listeners.

Validation

• flutter analyze lib/chat/chat_view.dart lib/chat/chat_view_model.dart
• git diff --check

[TiaraBasori]

Strict review found one remaining cache invalidation hole: mithkaChatHistoryCleared replaces messages and notifies listeners without advancing messageVersion, so _transcriptEntries can still reuse the old transcript when the list object/cache state lines up. I am patching this branch so every destructive transcript mutation invalidates the cache.